1. Field of the Invention:
The present invention relates to tendons for posttensioned prestressed concrete structures, which can be completely protected from corrosion without requiring grouting, can integrally be incorporated into prestressed concrete structures after being tensioned, and can easily be used for prestressing concrete structures, and also relates to a method of using such tendons.
2. Description of the Prior Art:
In the conventional posttensioning process for forming a prestressed concrete structure, sheaths are arranged prior to the placement of concrete, prestressing steel members such as steel bars, wires or strands are inserted in the sheaths after or before the concrete has set, and then the prestressing steel members are tensioned when the concrete has the desired strength. Then, a cement slurry or the like is injected under pressure into the sheaths for corrosion prevention and for integrally bonding the prestressing steel members to the concrete structure. The insertion of the prestressing steel members into the sheaths and the injection of the cement slurry or the like require very complicated work requiring a long time and much labor and increasing the cost of prestressed concrete structures. Furthermore, since, in most cases, the prestressing tendon is arranged in curvature, it is difficult to fill up the sheaths perfectly with the cement slurry or the like, and hence it is possible that the prestressing steel members in unfilled portions of the sheaths are corroded.
A method of eliminating such disadvantages of the conventional posttensioning process is proposed, for example, in Japanese Pat. Publication No. 53-47609 corresponding to U.S. Pat. No. 3,646,748, in which a prestressing member is formed by coating a steel material with a grease and encasing the steel material coated with the grease in a plastic case. This method completely prevents corrosion of the prestressing steel by grease or the like and makes injection of a cement slurry or the like unnecessary. However, the prestressing steel remains not bonded to the concrete structure after the same has been tensioned. Accordingly, when the prestressing tendon is overloaded temporarily, a load is concentrated on the fixed portions of the prestressing tendon to break the prestressing steel at the fixed portions. Since the prestressing steel is not bonded to the concrete structure, breakage of the prestressing steel, even at a single point thereon, affects the strength of the prestressed concrete structure entirely. Furthermore, the ultimate bending strength of a prestressed concrete structure having an unbonded prestressing tendon is lower than that of an equivalent prestressed concrete structure having a bonded prestressing tendon.
Austrian Pat. No. 201,280 and EP 219,284 propose structure of this general type but which do not teach or disclose a sheath. EP 129,976 shows corrugated sheaths in the drawings, but they are not seamless, and thus lack anticorrosion characteristics. U.S. Pat. No. 4,726,163 to Jacob shows an insulating material 9 in its drawings but this lacks a detailed explanation in the specification. U.S. Pat. No. 3,646,748 to Lano teaches a method of manufacturing a seamless sheath with a long span but does not teach a method of manufacturing a corrugated sheath. Therefore, the prior art is still characterized by difficulty in manufacturing a tendon with a corrugated sheath that is seamless and which has a long span.